1. Technical Field
This disclosure relates to an electrophotographic photoconductor capable of preventing a reduction in chargeability and occurrence of background smear and charging failure without causing an increase in potential at the exposed area even when continuously used over a long period of time, to an electrophotographic image forming apparatus such as a copier, printer, facsimile or a composite apparatus including a combination of functions processed by the above apparatuses each using the electrophotographic photoconductor, to an image forming method using the electrophotographic photoconductor, as well as to a process cartridge using the electrophotographic photoconductor.
2. Description of the Related Art
In recent years, from the viewpoints of saving office space and further widening business opportunities, and the like, electrophotographic image forming apparatus are desired to achieve high-speed performance, down-sizing, colorization, high-image quality performance as well as easy maintenance performance.
The achievement of these requirements is linked to improvements in physical properties and durability of electrophotographic photoconductors, and thus it is positioned as “the problem to solve” through developments of electrophotographic photoconductors. As one of the developments, from the perspective of improvements in easy maintenance performance of electrophotographic image forming apparatus, a reduction in replacement frequency of electrophotographic photoconductor is exemplified. The reduction in replacement frequency of electrophotographic photoconductor is closely relates to reducing major image defects caused by electrophotographic photoconductor as much as possible for a long period of time, and thus it means that providing long-life to electrophotographic photoconductor is required. This also relates to high-image quality performance of outputted images over a long period of time. Therefore, in recent years, lots of developments pertinent to providing longer life to electrophotographic photoconductors have been reported.
To achieve longer life of electrophotographic photoconductors, important is attached to improvement in durability to various hazards that could be incurred by electrophotographic photoconductors in image formation process. The hazards mentioned above are broadly classified into two types, i.e., mechanical hazards and electric hazards.
By way of example of mechanical hazards, there are hazards attributable to blade cleaner or blade cleaning device which is one of units for removing residual toner remaining on an electrophotographic photoconductor (in so-called toner cleaning process) after image transfer in image formation process. The blade cleaning device is a unit to forcedly remove residual toner from the surface of an electrophotographic photoconductor by making an elastic member (a so-called cleaning blade) in contact with the electrophotographic photoconductor, and exhibits remarkable toner removing ability in a narrow space, and thus the use of blade cleaning device is highly advantageous in downsizing of electrophotographic apparatus. On the other hand, such disadvantages of blade cleaning devices are pointed out that an electrophotographic photoconductor surface is abraded with an elastic member in a state of being in direct contact with the elastic member, and therefore a large mechanical stress is imposed on an electrophotographic photoconductor, and a layer functioning as an outermost surface layer of the photoconductor tends to wear. For this reason, in electrophotographic apparatus employing this cleaning process mode, abrasion of electrophotographic photoconductor surfaces often poses an impediment to providing longer life to the electrophotographic apparatus. To solve the problem, a technique is proposed in which high-hardness protective layers are vertically stacked (see Japanese Patent Application Laid-Open (JP-A) Nos. 5-181299, 2002-6526, 2002-82465, 2000-284514 and 2001-194813).
The following explains electrostatic hazards. In a typical image formation process, a charge is imparted to an electrophotographic photoconductor surface until the photoconductor is charged to a predetermined electric potential, and then the charge remaining on the electrophotographic photoconductor surface is removed by exposing the electrophotographic photoconductor to light. On this occasion, the charge passes through individual layers (e.g. a surface layer, charge generating layer, charge transporting layer, and intermediate layer) of the electrophotographic photoconductor, thereby an electrostatic stress is impressed on the electrophotographic photoconductor. Electrophotographic photoconductors which are widely diffused at the present time are mostly occupied by electrophotographic photoconductors consisting of organic materials. In the electrophotographic process where charging and removal of charge are repeatedly carried out, an organic material constituting an electrophotographic photoconductor gradually deteriorates by an electrostatic hazard to cause reductions in electrophotographic properties as typified by occurrence of charge trapping in layers of the electrophotographic photoconductor, a change in chargeability. It has been known that in particular a reduction in chargeability largely affects the quality of outputted image to cause a serious problem such as a reduction in image density, occurrence of background smear, and nonuniformity of image when continuously outputted.
There are various factors which are found to cause a reduction in chargeability of a photoconductor. For example, it is pointed out that electrostatic discharge product generated in charging process step in an image formation process has an influence on electrostatic properties of the photoconductor used (see Japanese Patent Application Laid-Open (JP-A) Nos. 9-26685, 2002-229241, and 2006-99028), whereby the bulk conductivity of bulk of a charge transporting layer and a surface layer of the photoconductor, leading to a reduction in chargeability of the photoconductor. To solve the problem, a technique is disclosed which prevents reduction in chargeability by adding an antioxidant to a charge transporting layer and a surface layer of an electrophotographic photoconductor (see Japanese Patent Application Laid-Open (JP-A) No. 2006-99028). Also, a charging technique is disclosed as a charging process, which causes less electrostatic discharge product (see Japanese Patent Application Laid-Open (JP-A) Nos. 9-26685 and 2002-229241). The use of the charging method in image formation process makes it possible to prevent reduction in chargeability of electrophotographic photoconductor caused by electrostatic discharge product.
As another factor of the reduction in chargeability, deterioration of an intermediate layer can be considered. Intermediate layers which are presently widely used are each formed in a dispersoid where inorganic fine particles are dispersed in a binder consisting of an organic resin and is said to preferably have a function of preventing electric charges from being injected from a support into a photosensitive layer and another function of transporting electric charges generated in the photosensitive layer to the support. For example, when the function of preventing electric charges from being injected from a support into a photosensitive layer does not satisfactorily work in an electrophotographic photoconductor, electric charges having a polarity opposite to the polarity of the electrophotographic photoconductor are injected from the support into the photosensitive layer on the occasion of charging the electrophotographic photoconductor, electric charges residing on the photoconductor surface are easily removed, causing a phenomenon that a desired charge amount is hardly obtained. When the function of transporting electric charges generated from the photosensitive layer to the support does not satisfactorily work, an increase in potential resulting from a reduction in charge generation easily occurs at an exposed area, and deficiency of chargeability easily takes place due to charge trapping in an intermediate layer. Various techniques to satisfy and maintain both of the two functions have been developed, however, these functions are generally in a conflicting relation each other, and it is very difficult to simultaneously satisfy and maintain both of the two functions. For instance, a technique is disclosed which improves a preventive effect against the injection of electric charges by employing a high-insulation layer as a structural layer of an intermediate layer (see Japanese Patent Application Laid-Open (JP-A) Nos. 3-45962 and 7-281463). It is reported that when this technique is used in an electrophotographic photoconductor, image defects, such as background smear, appeared due to a reduction in chargeability of the photoconductor are significantly reduced and outputted images can be kept high in quality even after repetitive use of the photoconductor. On the other hand, the function of transporting electric charges from a charge generating material to a support, which is another function played by an intermediate layer, becomes insufficient to easily cause an increase in potential at an exposed area, a charging defect and the like, and through repetitive use of an electrophotographic photoconductor, a phenomenon like an increase in potential at an exposed area and a charging defect becomes easily obvious.
Further, for example, a technique is proposed in which an electron transporting material is blended in an intermediate layer of an electrophotographic photoconductor whose surface is to be negatively charged in an image forming process (see Japanese Patent Application Laid-Open (JP-A) No. 2006-259141). According to the proposed technique, electric charges having a polarity opposite to the polarity of electric charges residing on the surface of a photoconductor (in this case, positive charge) is less injected into the photosensitive layer and a negative charge generated in the charge generating layer can be transported to the support, making it possible to obtain an intermediate layer which simultaneously satisfy both of the two functions described above. An electrophotographic photoconductor employing such an intermediate layer can exhibit highly superior electrophotographic properties in the initial stage of use of the photoconductor. This proposed technique, however, inconveniently causes degradation of electrophotographic properties by a repetitive electrostatic hazard. More specifically, an organic material constituting an intermediate layer tends to easily deteriorate due to a small amount of an electron transporting material exhibiting superior electron transfer property, contained in the intermediate layer and due to a repetitive electrostatic hazard, and further electron transporting material tends to easily form trapped charge under the influence of oxygen in the air.
Accordingly, an electrostatic photoconductor that can prevent electric charges from being injected from a support into a photosensitive layer without causing an increase in potential at an exposed area, and without substantially causing degradation of electrophotographic properties such as a degradation of chargeability even after use for a long period of time and enables to continuously obtain a high-quality image with less image defects; and an image forming method, and image forming apparatus and a process cartridge each using the electrophotographic photoconductor have not yet been obtained, and it is desired to quickly provide the photoconductor, image forming method, image forming apparatus and process cartridge.